1. Field of the Invention
The present invention relates to a charge pump rush current limiting circuit that limits a rush current in a charge pump circuit.
2. Description of the Related Art
In a conventional charge pump circuit, at the time of starting the supply of an input current from an input power supply to an output capacitor there is charqe in the output capacitor. Also, since the duty ratio of the charge pump under PFM control is kept constant, and a square wave is used in an oscillation pulse that allows a switch to turn on/off, a large rush current flows when the power supply is started.
However, in the conventional charge pump circuit, when the large rush current is generated when the power supply is started the result is that a voltage across the input power supply is dropped, and there is a fear that another circuit that is connected to the same input power supply as that of the charge pump malfunctions.
The present invention has been made to solve the above problem with the conventional device, and therefore an object of the present invention is to prevent another circuit connected to the same input power supply as that of a charge pump circuit from malfunctioning by limiting a rush current in such a manner that the operation of the charge pump circuit is stopped, and the voltage is charged up to a voltage of an output capacitor by the constant current circuit and thereafter a normal operation of the charge pump is started when a power supply is started, by limiting a rush current in such a manner that the duty ratio of the PFM operation is changed when the power supply is started, or alternatively, by employing a chopping wave or a sine wave as a pulse when a switch is turned on.
In order to achieve the above object, according to the present invention, a constant current circuit is disposed between an input power supply and an output capacitor, and when the power supply is started, the operation of the charge pump circuit is stopped, the output capacitor is charged up to a given voltage by the constant current circuit, and thereafter the normal operation of the charge pump is started. Alternatively, when the power turns on, the charge pump circuit is operated due to an oscillator circuit small in the duty ratio to limit the rush current, or a peak current value is averaged by using a chopping wave or a sine wave for a pulse that turns on the switch when the power is started.
In the conventional charge pump circuit, no charge exists in the output capacitor when the power supply is started. Therefore, there is a fear that the rush current flows in the output capacitor, the input voltage rapidly drops due to an impedance of the input power supply, and another circuit connected to the same input power supply malfunctions. In the present invention, the load capacitor is charged through the current limiting circuit without conducting the normal operation of the charge pump while the voltage across the output capacitor reaches a given voltage when the power supply is started. Therefore, the rush current does not flow until the voltage across the load capacitor reaches the given voltage, and the normal operation of the charge pump is conducted when the voltage across the load capacitor exceeds the given voltage. In this situation, a constant amount of charges has been already charged in the load capacitor, with the result that the amount of rush current becomes small as compared with a case in which no charge is charged in the output capacitor, to thereby suppress a drop of the supply voltage due to the impedance of the input power supply to be small and eliminate the malfunction of another circuit connected to the same input power supply.
An oscillator circuit having a small duty ratio is added to the circuit structure of the PFM control in the conventional DC/DC converter charge pump to integrate the oscillator circuits of two systems into the circuit. The charge pump circuit has an oscillator circuit which controls the charging operation of a pump capacitor by changing a frequency of a charging control signal while maintaining a constant pulse width thereof, and discharges the pumping capacitor to an output capacitor during discharging cycle. When the power supply is started, the circuit is operated by the oscillator circuit having a small duty ratio so that the amount of charges that are charged in the pumping capacitor and the output capacitor are reduced, to thereby make the rush current smaller than that started under the normal PFM control. Thereafter, when the output voltage reaches a given value, the oscillator circuit that is operating when the power supply is started is stopped, and then automatically changed over to the oscillator circuit having the normal duty ratio, to thereby conduct the PFM control.
Another pre-driver circuit in which a current is limited by a current limiting element is integrated into the conventional circuit structure of a pre-driver that turns on the switch of the charge pump so as to include the pre-driver circuits of the two systems integrated thereinto, and the switch is turned on by the pre-driver circuit in which a current is limited when the power supply is started, to thereby slowly turn on the switch, and a peak value of the rush current that is turned on by the normal square wave is decreased. Thereafter, when the output voltage reaches a given value, the pre-driver circuit that is operating when the power supply is started is stopped and automatically changed over to the normal pre-driver circuit, thereby turning on/off the switch.